Structural characterization of the outer core and the O-chain linkage region of lipopolysaccharide from Pseudomonas aeruginosa serotype O5

Reversed-phase HPLC based assay for selective and sensitive endotoxin quantification - part II

The impact of naturally occurring 3-deoxy-d-manno‑oct-2-ulsonic acid (Kdo) derivatives on endotoxin (ET) quantification was investigated for six ET standards. In our recently published chemical Kdo-DMB-LC ET assay (Bucsella et al., Anal. Methods, 2020, 12,4621) [1], the rare, ET specific sugar acid Kdo is used for ET quantification of S-type ETs. The ET content is calculated based on an external Kdo standard or a representative ET standard. In absence of a specific ET standard, the calculation is based on the reference standard ET (RSE) structure or on a worst-case scenario. This scenario overestimates the total ET content of typical S-type ET preparations by a factor of four. Mainly R-type ETs contain in addition to Kdo also Kdo-s non-stoichiometrically modified with phosphoethanolamine (PEtN), galactose (Gal) or L‑glycero-d-manno-heptose (Hep) in substantial quantities. These Kdo species are separated from the unmodified Kdo. All Kdo and Kdo species follow an exponential hydrolytic release from the ET core in dependence on the hydrolysis time. Hydrolysis kinetics for identical Kdo species are the same for all ET standards. Kdo-Gal was released fastest followed by unsubstituted Kdo, Kdo-PEtN, and Kdo-Hep. Between 90 and 150 min a plateau of maximum content is obtained for all Kdo-s. That allows in case of a representative ET standard, ET quantification based on the most present Kdo derivative, here mainly unsubstituted Kdo. If no representative ET standard is available Kdo and all Kdo species must be considered for ET quantification. With that the Kdo-DMB-LC assay is applicable for R- and S-type ETs.

Kinases on Double Duty: A Review of UniProtKB Annotated Bifunctionality within the Kinome

Phosphorylation facilitates the regulation of all fundamental biological processes, which has triggered extensive research of protein kinases and their roles in human health and disease. In addition to their phosphotransferase activity, certain kinases have evolved to adopt additional catalytic functions, while others have completely lost all catalytic activity. We searched the Universal Protein Resource Knowledgebase (UniProtKB) database for bifunctional protein kinases and focused on kinases that are critical for bacterial and human cellular homeostasis. These kinases engage in diverse functional roles, ranging from environmental sensing and metabolic regulation to immune-host defense and cell cycle control. Herein, we describe their dual catalytic activities and how they contribute to disease pathogenesis.

Synthesis of Aminooxy Glycoside Derivatives of the Outer Core Domain of Pseudomonas aeruginosa Lipopolysaccharide

Pseudomonas aeruginosa is a highly prevalent gram-negative bacterium that is becoming more difficult to treat because of increasing antibiotic resistance. As chemotherapeutic treatment options diminish, there is an increased need for vaccines. However, the creation of an effective P. aeruginosa vaccine has been elusive despite intensive efforts. Thus, new paradigms for vaccine antigens should be explored to develop effective vaccines. In these studies, we have focused on the synthesis of two L-rhamnose-bearing epitopes common to glycoforms I and II of the outer core domain of Pseudomonas aeruginosa lipopolysaccharide, α-L-Rha-(1→6)-α-D-Glc-(1→4)-α-D-GalN-(Ala)-α-aminooxy (3) and α-L-Rha-(1→3)-β-D-Glc-(1→3)-α-D-GalN-(Ala)-α-aminooxy (4), respectively. The target trisaccharides were both prepared starting from a suitably protected galactosamine glycoside, followed by successive deprotection and glycosylation with suitably protected D-glucose and L-rhamnose thioglycosides. Global deprotection resulted in the formation of targets 3 and 4 in 22 and 35% yield each. Care was required to modify basic reaction conditions to avoid early deprotection of the N-oxysuccinamido group. In summary, trisaccharides related to the L-rhamnose-bearing epitopes common to glycoforms I and II of the outer core domain of Pseudomonas aeruginosa lipopolysaccharide have been prepared as their aminooxy glycosides. The latter are expected to be useful in chemoselective oxime-based bioconjugation reactions to form Pseudomonas aeruginosa vaccines.

Host Defence Cryptides from Human Apolipoproteins: Applications in Medicinal Chemistry

Several eukaryotic proteins with defined physiological roles may act as precursors of cryptic bioactive peptides released upon protein cleavage by the host and/or bacterial proteases. Based on this, the term "cryptome" has been used to define the unique portion of the proteome encompassing proteins with the ability to generate bioactive peptides (cryptides) and proteins (crypteins) upon proteolytic cleavage. Hence, the cryptome represents a source of peptides with potential pharmacological interest. Among eukaryotic precursor proteins, human apolipoproteins play an important role, since promising bioactive peptides have been identified and characterized from apolipoproteins E, B, and A-I sequences. Human apolipoproteins derived peptides have been shown to exhibit antibacterial, anti-biofilm, antiviral, anti-inflammatory, anti-atherogenic, antioxidant, or anticancer activities in in vitro assays and, in some cases, also in in vivo experiments on animal models. The most interesting Host Defence Peptides (HDPs) identified thus far in human apolipoproteins are described here with a focus on their biological activities applicable to biomedicine. Altogether, reported evidence clearly indicates that cryptic peptides represent promising templates for the generation of new drugs and therapeutics against infectious diseases.

Effects of human antimicrobial cryptides identified in apolipoprotein B depend on specific features of bacterial strains

Cationic Host Defense Peptides (HDPs) are endowed with a broad variety of activities, including direct antimicrobial properties and modulatory roles in the innate immune response. Even if it has been widely demonstrated that bacterial membrane represents the main target of peptide antimicrobial activity, the molecular mechanisms underlying membrane perturbation by HDPs have not been fully clarified yet. Recently, two cryptic HDPs have been identified in human apolipoprotein B and found to be endowed with a broad-spectrum antimicrobial activity, and with anti-biofilm, wound healing and immunomodulatory properties. Moreover, ApoB derived HDPs are able to synergistically act in combination with conventional antibiotics, while being not toxic for eukaryotic cells. Here, by using a multidisciplinary approach, including time killing curves, Zeta potential measurements, membrane permeabilization assays, electron microscopy analyses, and isothermal titration calorimetry studies, the antimicrobial effects of ApoB cryptides have been analysed on bacterial strains either susceptible or resistant to peptide toxicity. Intriguingly, it emerged that even if electrostatic interactions between negatively charged bacterial membranes and positively charged HDPs play a key role in mediating peptide toxicity, they are strongly influenced by the composition of negatively charged bacterial surfaces and by defined extracellular microenvironments.

Structure and Conformation of Wild-Type Bacterial Lipopolysaccharide Layers at Air-Water Interfaces

The outer membrane of Gram-negative bacteria is of great scientific interest because it mediates the action of antimicrobial agents. The membrane surface is composed of lipopolysaccharide (LPS) molecules with negatively charged oligosaccharide headgroups. To a certain fraction, LPSs additionally display linear polysaccharides termed O-side chains (OSCs). Structural studies on bacterial outer surfaces models, based on LPS monolayers at air-water interfaces, have so far dealt only with rough mutant LPSs lacking these OSCs. Here, we characterize monolayers of wild-type LPS from Escherichia coli O55:B5 featuring strain-specific OSCs in the presence of defined concentrations of monovalent and divalent ions. Pressure-area isotherms yield insight into in-plane molecular interactions and monolayer elastic moduli. Structural investigations by x-ray and neutron reflectometry reveal the saccharide conformation and allow quantifying the area per molecule and the fraction of LPS molecules carrying OSCs. The OSC conformation is satisfactorily described by the self-consistent field theory for end-grafted polymer brushes. The monolayers exhibit a significant structural response to divalent cations, which goes beyond generic electrostatic screening.

A comparative study of the complete lipopolysaccharide structures and biosynthesis loci of Bordetella avium, B. hinzii, and B. trematum

A dozen species of human and animal pathogens have been described to date in the Bordetella genus, with the majority being respiratory tract pathogens. Bordetella avium lipopolysaccharides have been shown to be important virulence factors for this bird pathogen. B. hinzii is closely related to the B. avium species, but has also been isolated from humans. B. trematum is associated to ear and blood infections in humans. Its lipid A structure, the biological active moiety of LPS, was found to be closely related to those of B. avium and B. hinzii. It is important to unveil the subtle structural modifications orchestrated during the LPS biosynthetic pathway to better understand host adaptation. The present data are also important in the context of deciphering the virulence pathways of this important genus containing the major pathogens B. pertussis and B. parapertussis, responsible for whooping cough. We recently reported the isolated lipid A structures of the three presented species, following the previously identified O-chain structures. In the present study, we provide details on the free and O-chain-linked core oligosaccharides which were required to characterize the complete LPS structures. Data are presented here in relation to relevant biosynthesis genes. The present characterization of the three species is well illustrated by Matrix Assisted Laser Desorption Mass Spectrometry experiments, and data were obtained mainly on native LPS molecules for the first time.

Synthesis of Oligosaccharide Components of the Outer Core Domain of P. aeruginosa Lipopolysaccharide Using a Multifunctional Hydroquinone-Derived Reducing-End Capping Group

The synthesis of a trisaccharide (common to glycoform I and II) and a tetrasaccharide (common to glycoform I) from the outer core domain of Pseudomonas aeruginosa lipopolysaccharide (LPS) using a novel hydroquinone-based reducing-end capping group is reported. This multifunctional capping group was utilized as purification handle and was stable toward many common transformations in oligosaccharide synthesis. The access to outer-core LPS antigens with a TBDPS-protected hydroquinone (TPH) at the reducing end will be useful for glycan array and therapeutic glycoconjugate synthesis.

Review: Conserved and variable structural features in the lipopolysaccharide of Pseudomonas aeruginosa

The review is devoted to recent progress in the structural elucidation of the lipopolysaccharide of the bacterium Pseudomonas aeruginosa, including O-antigen biological repeats, core oligosaccharide, and lipid A. Data on biosynthesis, genetics and serology of the lipopolysaccharide isolated from various P. aeruginosa O-serogroups are discussed in relation to the chemical structures.

Lipopolysaccharide modification in Gram-negative bacteria during chronic infection

The Gram-negative bacterial lipopolysaccharide (LPS) is a major component of the outer membrane that plays a key role in host-pathogen interactions with the innate immune system. During infection, bacteria are exposed to a host environment that is typically dominated by inflammatory cells and soluble factors, including antibiotics, which provide cues about regulation of gene expression. Bacterial adaptive changes including modulation of LPS synthesis and structure are a conserved theme in infections, irrespective of the type or bacteria or the site of infection. In general, these changes result in immune system evasion, persisting inflammation and increased antimicrobial resistance. Here, we review the modifications of LPS structure and biosynthetic pathways that occur upon adaptation of model opportunistic pathogens (Pseudomonas aeruginosa, Burkholderia cepacia complex bacteria, Helicobacter pylori and Salmonella enterica) to chronic infection in respiratory and gastrointestinal sites. We also discuss the molecular mechanisms of these variations and their role in the host-pathogen interaction.

A Bacteriophage-Acquired O-Antigen Polymerase (Wzyβ) from P. aeruginosa Serotype O16 Performs a Varied Mechanism Compared to Its Cognate Wzyα

Pseudomonas aeruginosa is a Gram-negative bacterium that produces highly varied lipopolysaccharide (LPS) structures. The O antigen (O-Ag) in the LPS is synthesized through the Wzx/Wzy-dependent pathway where lipid-linked O-Ag repeats are polymerized by Wzy. Horizontal-gene transfer has been associated with O-Ag diversity. The O-Ag present on the surface of serotypes O5 and O16, differ in the intra-molecular bonds, α and β, respectively; the latter arose from the action of three genes in a serotype converting unit acquired from bacteriophage D3, including a β-polymerase (Wzy_β). To further our understanding of O-polymerases, the inner membrane (IM) topology of Wzy_β was determined using a dual phoA-lacZα reporter system wherein random 3′ gene truncations were localized to specific loci with respect to the IM by normalized reporter activities as determined through the ratio of alkaline phosphatase activity to β-galactosidase activity. The topology of Wzy_β developed through this approach was shown to contain two predominant periplasmic loops, PL3 (containing an RX₁₀G motif) and PL4 (having an O-Ag ligase superfamily motif), associated with inverting glycosyltransferase reaction. Through site-directed mutagenesis and complementation assays, residues Arg²⁵⁴, Arg²⁷⁰, Arg²⁷², and His³⁰⁰ were found to be essential for Wzy_β function. Additionally, like-charge substitutions, R254K and R270K, could not complement the wzy_β knockout, highlighting the essential guanidium side group of Arg residues. The O-Ag ligase domain is conserved among heterologous Wzy proteins that produce β-linked O-Ag repeat units. Taking advantage of the recently obtained whole-genome sequence of serotype O16 a candidate promoter was identified. Wzy_β under its native promoter was integrated in the PAO1 genome, which resulted in simultaneous production of α- and β-linked O-Ag. These observations established that members of Wzy-like family consistently exhibit a dual-periplasmic loops topology, and identifies motifs that are plausible to be involved in enzymatic activities. Based on these results, the phage-derived Wzy_β utilizes a different reaction mechanism in the P. aeruginosa host to avoid self-inhibition during serotype conversion.

Lipopolysaccharides from Commensal and Opportunistic Bacteria: Characterization and Response of the Immune System of the Host Sponge Suberites domuncula

Marine sponges harbor a rich bacterioflora with which they maintain close relationships. However, the way these animals make the distinction between bacteria which are consumed to meet their metabolic needs and opportunistic and commensal bacteria which are hosted is not elucidated. Among the elements participating in this discrimination, bacterial cell wall components such as lipopolysaccharides (LPS) could play a role. In the present study, we investigated the LPS chemical structure of two bacteria associated with the sponge Suberites domuncula: a commensal Endozoicomonas sp. and an opportunistic Pseudoalteromonas sp. Electrophoretic patterns indicated different LPS structures for these bacteria. The immunomodulatory lipid A was isolated after mild acetic acid hydrolysis. The electrospray ionization ion-trap mass spectra revealed monophosphorylated molecules corresponding to tetra- and pentaacylated structures with common structural features between the two strains. Despite peculiar structural characteristics, none of these two LPS influenced the expression of the macrophage-expressed gene S. domuncula unlike the Escherichia coli ones. Further research will have to include a larger number of genes to understand how this animal can distinguish between LPS with resembling structures and discriminate between bacteria associated with it.

Modulating LPS signal transduction at the LPS receptor complex with synthetic Lipid A analogues

Sepsis, defined as a clinical syndrome brought about by an amplified and dysregulated inflammatory response to infections, is one of the leading causes of death worldwide. Despite persistent attempts to develop treatment strategies to manage sepsis in the clinical setting, the basic elements of treatment have not changed since the 1960s. As such, the development of effective therapies for reducing inflammatory reactions and end-organ dysfunction in critically ill patients with sepsis remains a global priority. Advances in understanding of the immune response to sepsis provide the opportunity to develop more effective pharmaceuticals. This article details current information on the modulation of the lipopolysaccharide (LPS) receptor complex with synthetic Lipid A mimetics. As the initial and most critical event in sepsis pathophysiology, the LPS receptor provides an attractive target for antisepsis agents. One of the well-studied approaches to sepsis therapy involves the use of derivatives of Lipid A, the membrane-anchor portion of an LPS, which is largely responsible for its endotoxic activity. This article describes the structural and conformational requirements influencing the ability of Lipid A analogues to compete with LPS for binding to the LPS receptor complex and to inhibit the induction of the signal transduction pathway by impairing LPS-initiated receptor dimerization.

Identification of the linkage between A-polysaccharide and the core in the A-lipopolysaccharide of Porphyromonas gingivalis W50

Porphyromonas gingivalis synthesizes two lipopolysaccharides (LPSs), O-LPS and A-LPS. The structure of the core oligosaccharide (OS) of O-LPS and the attachment site of the O-polysaccharide (O-PS) repeating unit [ → 3)-α-D-Galp-(1 → 6)-α-D-Glcp-(1 → 4)-α-L-Rhap-(1 → 3)-β-D-GalNAcp-(1 → ] to the core have been elucidated using the ΔPG1051 (WaaL, O-antigen ligase) and ΔPG1142 (Wzy, O-antigen polymerase) mutant strains, respectively. The core OS occurs as an "uncapped" glycoform devoid of O-PS and a "capped" glycoform that contains the attachment site of O-PS via β-d-GalNAc at position O-3 of the terminal α-(1 → 3)-linked mannose (Man) residue. In this study, the attachment site of A-PS to the core OS was determined based on structural analysis of SR-type LPS (O-LPS and A-LPS) isolated from a P. gingivalis ΔPG1142 mutant strain by extraction with aqueous hot phenol to minimize the destruction of A-LPS. Application of one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy in combination with methylation analysis showed that the A-PS repeating unit is linked to a nonterminal α-(1 → 3)-linked Man of the "capped core" glycoform of outer core OS at position O-4 via a → 6)-[α-D-Man-α-(1 → 2)-α-D-Man-1-phosphate → 2]-α-D-Man-(1 → motif. In order to verify that O-PS and A-PS are attached to almost identical core glycoforms, we identified a putative α-mannosyltransferase (PG0129) in P. gingivalis W50 that may be involved in the formation of core OS. Inactivation of PG0129 led to the synthesis of deep-R-type LPS with a truncated core that lacks α-(1 → 3)-linked mannoses and is devoid of either O-PS or A-PS. This indicated that PG0129 is an α-1,3-mannosyltransferase required for synthesis of the outer core regions of both O-LPS and A-LPS in P. gingivalis.

IMPORTANCE

Porphyromonas gingivalis, a Gram-negative anaerobe, is considered to be an important etiologic agent in periodontal disease, and among the virulence factors produced by the organism are two lipopolysaccharides (LPSs), O-LPS and A-LPS. The structures of the O-PS and A-PS repeating units, the core oligosaccharide (OS), and the linkage of the O-PS repeating unit to the core OS in O-LPS have been elucidated by our group. It is important to establish whether the attachment site of the A-PS repeating unit to the core OS in A-LPS is similar to or differs from that of the O-PS repeating unit in O-LPS. As part of understanding the biosynthetic pathway of the two LPSs in P. gingivalis, PG0129 was identified as an α-mannosyltransferase that is involved in the synthesis of the outer core regions of both O-LPS and A-LPS.

Novel regio- and stereoselective phosphonyl radical addition to glycals promoted by Mn(ii)–air: syntheses of 1,2-dideoxy 2-C-diphenylphosphinylglycopyranosides

The novel Mn(ii)–air promoted radical reaction of diphenylphosphine oxide with various glycals in excellent regio- and stereoselectivities generated 1,2-dideoxy-2-C-diphenylphosphinylglycopyranosides.

The WaaL O-antigen lipopolysaccharide ligase has features in common with metal ion-independent inverting glycosyltransferases

WaaL is a membrane enzyme that catalyzes a key step in lipopolysaccharide (LPS) synthesis: the glycosidic bonding of a sugar at the proximal end of the undecaprenyl-diphosphate (Und-PP) O-antigen with a terminal sugar of the lipid A-core oligosaccharide (OS). Utilizing an in vitro assay, we demonstrate here that ligation with purified Escherichia coli WaaL occurs without adenosine-5'-triphosphate (ATP) and magnesium ions. Furthermore, E. coli and Pseudomonas aeruginosa WaaL proteins cannot catalyze ATP hydrolysis in vitro. We also show that a lysine substitution of the arginine (Arg)-215 residue renders an active protein, whereas WaaL mutants with alanine replacements in the periplasmic-exposed residues Arg-215, Arg-288 and histidine (His)-338 and also the membrane-embedded aspartic acid-389 are nonfunctional. An in silico approach, combining predicted topological information with the analysis of sequence conservation, confirms the importance of a positive charge at the small periplasmic loop of WaaL, since an Arg corresponding to Arg-215 was found at a similar position in all the WaaL homologs. Also, a universally conserved H[NSQ]X(9)GXX[GTY] motif spanning the C-terminal end of the predicted large periplasmic loop and the membrane boundary of the transmembrane helix was identified. The His residue in this motif corresponds to His-338. A survey of LPS structures in which the linkage between O-antigen and lipid A-core OS was elucidated reveals that it is always in the β-configuration, whereas the sugars bound to Und-PP are in the α-configuration. Together, our biochemical and in silico data argue that WaaL proteins use a common reaction mechanism and share features of metal ion-independent inverting glycosyltransferases.

Lipopolysaccharide (LPS) inner-core phosphates are required for complete LPS synthesis and transport to the outer membrane in Pseudomonas aeruginosa PAO1

Gram-negative outer membrane (OM) integrity is maintained in part by Mg²⁺ cross-links between phosphates on lipid A and on core sugars of adjacent lipopolysaccharide (LPS) molecules. In contrast to other Gram-negative bacteria, waaP, encoding an inner-core kinase, could not be inactivated in Pseudomonas aeruginosa. To examine this further, expression of the kinases WaaP or WapP/WapQ/PA5006 was placed under the control of the arabinose-regulated pBAD promoter. Growth of these strains was arabinose dependent, confirming that core phosphorylation is essential in P. aeruginosa. Transmission electron micrographs of kinase-depleted cells revealed marked invaginations of the inner membrane. SDS-PAGE of total LPS from WaaP-depleted cells showed accumulation of a fast-migrating band. Mass spectrometry (MS) analysis revealed that LPS from these cells exhibits a unique truncated core consisting of two 3-deoxy-d-manno-octulosonic acids (Kdo), two l-glycero-d-manno-heptoses (Hep), and one hexose but completely devoid of phosphates, indicating that phosphorylation by WaaP is necessary for subsequent core phosphorylations. MS analysis of lipid A from WaaP-depleted cells revealed extensive 4-amino-4-deoxy-l-arabinose modification. OM prepared from these cells by Sarkosyl extraction of total membranes or by sucrose density gradient centrifugation lacked truncated LPS. Instead, truncated LPS was detected in the inner membrane fractions, consistent with impaired transport/assembly of this species into the OM.

Gram-negative bacteria have an outer membrane (OM) comprised of a phospholipid inner leaflet and a lipopolysaccharide (LPS) outer leaflet. The OM protects cells from toxic molecules and is important for survival during infection. The LPS core kinase gene waaP can be deleted in several Gram-negative bacteria but not in Pseudomonas aeruginosa. We used a controlled-expression system to deplete WaaP directly in P. aeruginosa cells, which halted growth. WaaP depletion also caused gross changes in cell morphology and led to the accumulation of an aberrant LPS lacking several core sugars and all core phosphates. The aberrant LPS failed to reach the OM, suggesting that WaaP is essential in P. aeruginosa because it is required to produce the full-length LPS that is recognized by the OM transport/assembly machinery in this organism. Therefore, WaaP may constitute a good target for the development of novel antipseudomonal agents.

The lipopolysaccharide of the mastitis isolate Escherichia coli strain 1303 comprises a novel O-antigen and the rare K-12 core type

Mastitis represents one of the most significant health problems of dairy herds. The two major causative agents of this disease are Escherichia coli and Staphylococcus aureus. Of the first, its lipopolysaccharide (LPS) is thought to play a prominent role during infection. Here, we report the O-antigen (OPS, O-specific polysaccharide) structure of the LPS from bovine mastitis isolate E. coli 1303. The structure was determined utilizing chemical analyses, mass spectrometry, and 1D and 2D NMR spectroscopy methods. The O-repeating unit was characterized as -[→4)-β-d-Quip3NAc-(1→3)-α-l-Fucp2OAc-(1→4)-β-d-Galp-(1→3)-α-d-GalpNAc-(1→]- in which the O-acetyl substitution was non-stoichiometric. The nucleotide sequence of the O-antigen gene cluster of E. coli 1303 was also determined. This cluster, located between the gnd and galF genes, contains 13 putative open reading frames, most of which represent unknown nucleotide sequences that have not been described before. The O-antigen of E. coli 1303 was shown to substitute O-7 of the terminal ld-heptose of the K-12 core oligosaccharide. Interestingly, the non-OPS-substituted core oligosaccharide represented a truncated version of the K-12 outer core – namely terminal ld-heptose and glucose were missing; however, it possessed a third Kdo residue in the inner core. On the basis of structural and genetic data we show that the mastitis isolate E. coli 1303 represents a new serotype and possesses the K-12 core type, which is rather uncommon among human and bovine isolates.

Mucosal vaccination with a multivalent, live-attenuated vaccine induces multifactorial immunity against Pseudomonas aeruginosa acute lung infection

Many animal studies investigating adaptive immune effectors important for protection against Pseudomonas aeruginosa have implicated opsonic antibody to the antigenically variable lipopolysaccharide (LPS) O antigens as a primary effector. However, active and passive vaccination of humans against these antigens has not shown clinical efficacy. We hypothesized that optimal immunity would require inducing multiple immune effectors targeting multiple bacterial antigens. Therefore, we evaluated a multivalent live-attenuated mucosal vaccination strategy in a murine model of acute P. aeruginosa pneumonia to assess the contributions to protective efficacy of various bacterial antigens and host immune effectors. Vaccines combining 3 or 4 attenuated strains having different LPS serogroups were associated with the highest protective efficacy compared to vaccines with fewer components. Levels of opsonophagocytic antibodies, which were directed not only to the LPS O antigens but also to the LPS core and surface proteins, correlated with protective immunity. The multivalent live-attenuated vaccines overcame prior problems involving immunologic interference in the development of O-antigen-specific antibody responses when closely related O antigens were combined in multivalent vaccines. Antibodies to the LPS core were associated with in vitro killing and in vivo protection against strains with O antigens not expressed by the vaccine strains, whereas antibodies to the LPS core and surface proteins augmented the contribution of O-antigen-specific antibodies elicited by vaccine strains containing a homologous O antigen. Local CD4 T cells in the lung also contributed to vaccine-based protection when opsonophagocytic antibodies to the challenge strain were absent. Thus, multivalent live-attenuated vaccines elicit multifactorial protective immunity to P. aeruginosa lung infections.

Structural investigation of bacterial lipopolysaccharides by mass spectrometry and tandem mass spectrometry

Mass spectrometric studies are now playing a leading role in the elucidation of lipopolysaccharide (LPS) structures through the characterization of antigenic polysaccharides, core oligosaccharides and lipid A components including LPS genetic modifications. The conventional MS and MS/MS analyses together with CID fragmentation provide additional structural information complementary to the previous analytical experiments, and thus contribute to an integrated strategy for the simultaneous characterization and correct sequencing of the carbohydrate moiety.

Review: Lipopolysaccharide biosynthesis in Pseudomonas aeruginosa

Pseudomonas aeruginosa causes serious nosocomial infections, and an important virulence factor produced by this organism is lipopolysaccharide (LPS). This review summarizes knowledge about biosynthesis of all three structural domains of LPS - lipid A, core oligosaccharide, and O polysaccharides. In addition, based on similarities with other bacterial species, this review proposes new hypothetical pathways for unstudied steps in the biosynthesis of P. aeruginosa LPS. Lipid A biosynthesis is discussed in relation to Escherichia coli and Salmonella, and the biosyntheses of core sugar precursors and core oligosaccharide are summarised. Pseudomonas aeruginosa attaches a Common Polysaccharide Antigen and O-Specific Antigen polysaccharides to lipid A-core. Both forms of O polysaccharide are discussed with respect to their independent synthesis mechanisms. Recent advances in understanding O-polysaccharide biosynthesis since the last major review on this subject, published nearly a decade ago, are highlighted. Since P. aeruginosa O polysaccharides contain unusual sugars, sugar-nucleotide biosynthesis pathways are reviewed in detail. Knowledge derived from detailed studies in the O5, O6 and O11 serotypes is applied to predict biosynthesis pathways of sugars in poorly-studied serotypes, especially O1, O4, and O13/O14. Although further work is required, a full understanding of LPS biosynthesis in P. aeruginosa is almost within reach.

Structural analysis of the core region of O-lipopolysaccharide of Porphyromonas gingivalis from mutants defective in O-antigen ligase and O-antigen polymerase

Porphyromonas gingivalis synthesizes two lipopolysaccharides (LPSs), O-LPS and A-LPS. Here, we elucidate the structure of the core oligosaccharide (OS) of O-LPS from two mutants of P. gingivalis W50, Delta PG1051 (WaaL, O-antigen ligase) and Delta PG1142 (O-antigen polymerase), which synthesize R-type LPS (core devoid of O antigen) and SR-type LPS (core plus one repeating unit of O antigen), respectively. Structural analyses were performed using one-dimensional and two-dimensional nuclear magnetic resonance spectroscopy in combination with composition and methylation analysis. The outer core OS of O-LPS occurs in two glycoforms: an "uncapped core," which is devoid of O polysaccharide (O-PS), and a "capped core," which contains the site of O-PS attachment. The inner core region lacks L(D)-glycero-D(l)-manno-heptosyl residues and is linked to the outer core via 3-deoxy-D-manno-octulosonic acid, which is attached to a glycerol residue in the outer core via a monophosphodiester bridge. The outer region of the "uncapped core" is attached to the glycerol and is composed of a linear alpha-(1-->3)-linked d-Man OS containing four or five mannopyranosyl residues, one-half of which are modified by phosphoethanolamine at position 6. An amino sugar, alpha-D-allosamine, is attached to the glycerol at position 3. In the "capped core," there is a three- to five-residue extension of alpha-(1-->3)-linked Man residues glycosylating the outer core at the nonreducing terminal residue. beta-D-GalNAc from the O-PS repeating unit is attached to the nonreducing terminal Man at position 3. The core OS of P. gingivalis O-LPS is therefore a highly unusual structure, and it is the basis for further investigation of the mechanism of assembly of the outer membrane of this important periodontal bacterium.

Genetic and biochemical analyses of the Pseudomonas aeruginosa Psl exopolysaccharide reveal overlapping roles for polysaccharide synthesis enzymes in Psl and LPS production

Exopolysaccharides contribute significantly to attachment and biofilm formation in the opportunisitc pathogen Pseudomonas aeruginosa. The Psl polysaccharide, which is synthesized by the polysaccharide synthesis locus (psl), is required for biofilm formation in non-mucoid strains that do not rely on alginate as the principal biofilm polysaccharide. In-frame deletion and complementation studies of individual psl genes revealed that 11 psl genes, pslACDEFGHIJKL, are required for Psl production and surface attachment. We also present the first structural analysis of the psl-dependent polysaccharide, which consists of a repeating pentasaccharide containing d-mannose, d-glucose and l-rhamnose: [See text]. In addition, we identified the sugar nucleotide precursors involved in Psl generation and demonstrated the requirement for GDP-d-mannose, UDP-d-glucose and dTDP-l-rhamnose in Psl production and surface attachment. Finally, genetic analyses revealed that wbpW restored Psl production in a pslB mutant and pslB promoted A-band LPS synthesis in a wbpW mutant, indicating functional redundancy and overlapping roles for these two enzymes. The structural and genetic data presented here provide a basis for further investigation of the Psl proteins and potential roles for Psl in the biology and pathogenesis of P. aeruginosa.

Calcium ions induce collapse of charged O-side chains of lipopolysaccharides from Pseudomonas aeruginosa

Lipopolysaccharide (LPS) monolayers deposited on planar, hydrophobic substrates were used as a defined model of outer membranes of Pseudomonas aeruginosa strain dps 89. To investigate the influence of ions on the (out-of-plane) monolayer structure, we measured specular X-ray reflectivity at high energy (22 keV) to ensure transmission through water. Electron density profiles were reconstructed from the reflectivity curves, and they indicate that the presence of Ca(2+) ions induces a significant change in the conformation of the charged polysaccharide head groups (O-side chains). Monte Carlo simulations based on a minimal computer model of LPS molecules allow for the modelling of 100 or more molecules over 10(-3) s and theoretically explained the tendency found by experiments.

Truncation in the core oligosaccharide of lipopolysaccharide affects flagella-mediated motility in Pseudomonas aeruginosa PAO1 via modulation of cell surface attachment

In many Gram-negative bacterial species, rough strains producing truncated lipopolysaccharide (LPS) generally exhibit defects in motility compared with smooth strains. However, the role that LPS plays in bacterial motility is not well understood. The goal of this study was to examine the relationship between LPS defects and motility of Pseudomonas aeruginosa. P. aeruginosa wild-type strain PAO1 and three isogenic mutants with defects in the rmlC, migA and wapR genes and producing truncated core oligosaccharide were investigated in terms of motility, attachment to glass and flagella expression. Compared with the wild-type, the three mutants showed significant retardation in both swarming motility on 0.5 % soft-agar plates and swimming motility on 0.3 % soft-agar plates. Moreover, attachment to abiotic surfaces was observed to be stronger in these mutants. The assembly of flagella appeared to be intact in these strains and the ability of individual cells to swim was unaffected. Flagellin proteins prepared from mutants rmlC and rmd, defective in the production of TDP-l-rhamnose and GDP-d-rhamnose, respectively, were compared and a change in molecular mass was observed only in the rmlC mutant. These data indicated that l-rhamnose, and not its enantiomer, d-rhamnose, is incorporated into the flagellin glycan of P. aeruginosa PAO1. The nucleotide-activated sugar precursor TDP-l-rhamnose is therefore shared between LPS biosynthesis and flagellin glycosylation in P. aeruginosa PAO1. Our results suggest that although biochemical precursors are shared by LPS and flagellin glycan biosynthesis, LPS truncations probably alter flagella-mediated motility in P. aeruginosa by modulating cell-surface attachment but not flagella synthesis.

Functional characterization of MigA and WapR: putative rhamnosyltransferases involved in outer core oligosaccharide biosynthesis of Pseudomonas aeruginosa

Pseudomonas aeruginosa lipopolysaccharide (LPS) contains two glycoforms of core oligosaccharide (OS); one form is capped with O antigen through an alpha-1,3-linked L-rhamnose (L-Rha), while the other is uncapped and contains an alpha-1,6-linked L-Rha. Two genes in strain PAO1, wapR (PA5000) and migA (PA0705), encode putative glycosyltransferases associated with core biosynthesis. We propose that WapR and MigA are the rhamnosyltransferases responsible for the two linkages of L-Rha to the core. Knockout mutants with mutations in both genes were generated. The wapR mutant produced LPS lacking O antigen, and addition of wapR in trans complemented this defect. The migA mutant produced LPS with a truncated outer core and showed no reactivity to outer core-specific monoclonal antibody (MAb) 5C101. Complementation of this mutant with migA restored reactivity of the LPS to MAb 5C101. Interestingly, LPS from the complemented migA strain was not reactive to MAb 18-19 (specific for the core-plus-one O repeat). This was due to overexpression of MigA in the complemented strain that caused an increase in the proportion of the uncapped core OS, thereby decreasing the amount of the core-plus-one O repeat, indicating that MigA has a regulatory role. The structures of LPS from both mutants were elucidated using nuclear magnetic resonance spectroscopy and mass spectrometry. The capped core of the wapR mutant was found to be truncated and lacked alpha-1,3-L-Rha. In contrast, uncapped core OS from the migA mutant lacked alpha-1,6-L-Rha. These results provide evidence that WapR is the alpha-1,3-rhamnosyltransferase, while MigA is the alpha-1,6-rhamnosyltransferase.

Quantifying the adhesion and interaction forces between Pseudomonas aeruginosa and natural organic matter

Atomic force microscopy (AFM) was used to characterize interactions between natural organic matter (NOM), and glass or bacteria. Poly(methacrylic acid) (PMA), soil humic Acid (SHA), and Suwannee River humic Acid (SRHA), were adsorbed to silica AFM probes. Adhesion forces (Fadh) for the interaction of organic-probes and glass slides correlated with organic molecular weight (MW), but not with radius of the organic aggregate (R), charge density (Q), or zeta potential (zeta). Two Pseudomonas aeruginosa strains with different lipopolysaccharides (LPS) were chosen: PAO1 (A+B+), whose LPS have common antigen (A-band) + O-antigen (B-band); and mutant AK1401 (A+B-). Fadh between bacteria and organics correlated with organic MW, R, and Q, but not zeta. PAO1 had lower Fadh with silica than NOM, which was attributed to negative charges from the B-band polymers causing electrostatic repulsion. AK1401 adhered stronger to silica than to the organics, perhaps because the absence of the B-band exposed underlying positively charged proteins. DLVO calculations could not explain the differences in the two bacteria or predict qualitative or quantitative trends in interaction forces in these systems. Molecular-level information from AFM studies can bring us closer to understanding the complex nature of bacterial-NOM interactions.

Characterization of the lipopolysaccharide from a wbjE mutant of the serogroup O11 Pseudomonas aeruginosa strain, PA103

The lipopolysaccharide (LPS) of a wbjE mutant of Pseudomonas aeruginosa PA103, a serogroup O11 strain consists of both high and low molecular weight (HMW and LMW) LPSs. The HMW LPS consisted exclusively of rhamnan A-band LPS and no B-band LPS was detected in the wbjE mutant. Interestingly, the LMW LPS from the wbjE mutant showed that it contained a variety of oligosaccharides, each with two or three phosphate groups present as mono- or pyrophosphates. These oligosaccharides consisted of the complete core octasaccharide. The GalN residue was present as an N-acetylated residue in all of these oligosaccharides except the tetrasaccharide in which it is present as an N-alanylated residue. None of these oligosaccharides contained either a d- or l-FucpNAc residue. These results are discussed with regard to the role of wbjE in the biosynthesis of P. aeruginosa PA103 B-band LPS.

Natriuretic peptides affect Pseudomonas aeruginosa and specifically modify lipopolysaccharide biosynthesis

Natriuretic peptides of various forms are present in animals and plants, and display structural similarities to cyclic antibacterial peptides. Pretreatment of Pseudomonas aeruginosa PAO1 with brain natriuretic peptide (BNP) or C-type natriuretic peptide (CNP) increases bacterium-induced glial cell necrosis. In eukaryotes, natriuretic peptides act through receptors coupled to cyclases. We observed that stable analogs of cAMP (dibutyryl cAMP) and cGMP (8-bromo-cGMP) mimicked the effect of brain natriuretic peptide and CNP on bacteria. Further evidence for the involvement of bacterial cyclases in the regulation of P. aeruginosa PAO1 cytotoxicity by natriuretic peptides is provided by the observed doubling of intrabacterial cAMP concentration after exposure to CNP. Lipopolysaccharide (LPS) extracted from P. aeruginosa PAO1 treated with both dibutyryl cAMP and 8-bromo-cGMP induces higher levels of necrosis than LPS extracted from untreated bacteria. Capillary electrophoresis and MALDI-TOF MS analysis have shown that differences in LPS toxicity are due to specific differences in the structure of the macromolecule. Using a strain deleted in the vfr gene, we showed that the Vfr protein is essential for the effect of natriuretic peptides on P. aeruginosa PAO1 virulence. These data support the hypothesis that P. aeruginosa has a cyclic nucleotide-dependent natriuretic peptide sensor system that may affect virulence by activating the expression of Vfr and LPS biosynthesis.

Pseudomonas aeruginosa Psl is a galactose- and mannose-rich exopolysaccharide

The Pseudomonas aeruginosa polysaccharide synthesis locus (psl) is predicted to encode an exopolysaccharide which is critical for biofilm formation. Here we used chemical composition analyses and mannose- or galactose-specific lectin staining, followed by confocal laser scanning microscopy and electron microscopy, to show that Psl is a galactose-rich and mannose-rich exopolysaccharide.

Pseudomonas aeruginosa lipopolysaccharide: a major virulence factor, initiator of inflammation and target for effective immunity

Pseudomonas aeruginosa is one of the most important bacterial pathogens encountered by immunocompromised hosts and patients with cystic fibrosis (CF), and the lipopolysaccharide (LPS) elaborated by this organism is a key factor in virulence as well as both innate and acquired host responses to infection. The molecule has a fair degree of heterogeneity in its lipid A and O-antigen structure, and elaborates two different outer-core glycoforms, of which only one is ligated to the O-antigen. A close relatedness between the chemical structures and genes encoding biosynthetic enzymes has been established, with 11 major O-antigen groups identified. The lipid A can be variably penta-, hexa- or hepta-acylated, and these isoforms have differing potencies when activating host innate immunity via binding to Toll-like receptor 4 (TLR4). The O-antigen is a major target for protective immunity as evidenced by numerous animal studies, but attempts, to date, to produce a human vaccine targeting these epitopes have not been successful. Newer strategies employing live attenuated P. aeruginosa, or heterologous attenuated bacteria expressing P. aeruginosa O-antigens are potential means to solve some of the existing problems related to making a P. aeruginosa LPS-specific vaccine. Overall, there is now a large amount of information available about the genes and enzymes needed to produce the P. aeruginosa LPS, detailed chemical structures have been determined for the major O-antigens, and significant biologic and immunologic studies have been conducted to define the role of this molecule in virulence and immunity to P. aeruginosa infection.

Neutron Diffraction Study ofPseudomonasaeruginosaLipopolysaccharide Bilayers

Lipopolysaccharides (LPSs) are a major class of macromolecules populating the surface of Gram-negative bacteria. They contribute significantly to the bacterium's surface properties and play a crucial role in regulating the permeability of its outer membrane. Here, we report on neutron diffraction studies performed on aligned, self-assembled bilayers of LPS isolated from Pseudomonas aeruginosa PAO1. This LPS system is comprised of a mixture of rough and smooth A-band and B-band LPS, similar to that naturally found in P. aeruginosa. Temperature scans were conducted at various levels of hydration, and the phases adopted by LPS, along with their corresponding transition temperatures, have been identified. Because of LPS's chemical heterogeneity, the gel-to-liquid-crystalline transition was continuous and not abrupt as commonly observed in single-component phospholipid systems. From the construction of one-dimensional scattering length density profiles, we find that water penetrates into the hydrocarbon region up to and including the center of liquid-crystalline LPS bilayers. This permeability to water also extends to bilayers in the continuous phase transition region and could have far-reaching implications as to how small molecules penetrate the outer membrane of Gram-negative bacteria.

Structures of the core oligosaccharide and O-units in the R- and SR-type lipopolysaccharides of reference strains of Pseudomonas aeruginosa O-serogroups

Highly phosphorylated core oligosaccharides and those substituted with one O-antigen repeating unit were obtained by mild acid degradation or strong alkaline hydrolysis of lipopolysaccharide samples from 23 reference strains representing all Pseudomonas aeruginosa O-serogroups. Studies by high-resolution electrospray ionization mass spectrometry and two-dimensional NMR spectroscopy revealed both conserved and variable structural features of the lipopolysaccharides of various O-serogroups. The upstream terminal saccharide of the O-antigen, which contributes most to the immunospecificity of the bacteria, was defined in 11 from a total of 13 O-serogroups. The data obtained link together the known biosynthesis pathways, genetics and serology of the P. aeruginosa lipopolysaccharide.

Structures of the biological repeating units in the O-chain polysaccharides ofHafnia alveistrains having a typical lipopolysaccharide outer core region

Earlier, the structures of the O-chain polysaccharides of the lipopolysaccharides (LPS) of a number of Hafnia alvei strains have been established. However, it remained unknown, which is the first and the last monosaccharide of the O-chain. This is defined by the structure of the so-called biological repeating unit (O-unit), which is pre-assembled and then polymerised in the course of biosynthesis of bacterial polysaccharides by the Wzy-dependent pathway. Now we report on the structures of the O-units in 10 H. alvei strains. The LPS were cleaved by mild acid hydrolysis and oligosaccharide fractions IIIa and IIIb were isolated by gel chromatography subsequently on Sephadex G-50 and BioGel P-2 and studied by methylation analysis and NMR spectroscopy. Fraction IIIb was found to represent the core oligosaccharide containing a terminal upstream alpha-d-Glc-(1-->3)-alpha-d-Glc or alpha-d-Gal-(1-->3)-alpha-d-Glc disaccharide in the outer region that is typical of H. alvei. Fraction IIIa consists of the LPS core with one O-unit linked by a 3-substituted beta-d-GalNAc residue (in strains PCM 1189 and PCM 1546) or a 3-substituted beta-d-GlcNAc residue (in the other strains studied). In most strains examined the beta-configuration of the d-GlcNAc linkage in the first O-unit attached to the core is the same and in some strains is opposite to that found in the interior O-units of the O-chain polysaccharide. Various monosaccharides, including d-Glc, d-Gal, d-GlcA and acyl derivatives of 3-amino-3,6-dideoxy-d-glucose or 4-amino-4,6-dideoxy-d-glucose, occupy the non-reducing end of the O-unit.

Structure of the core oligosaccharide of a rough-type lipopolysaccharide of Pseudomonas syringae pv. phaseolicola

The core structure of the lipopolysaccharide (LPS) isolated from a rough strain of the phytopathogenic bacterium Pseudomonas syringae pv. phaseolicola, GSPB 711, was investigated by sugar and methylation analyses, Fourier transform ion-cyclotron resonance ESI MS, and one- and two-dimensional 1H-, 13C- and 31P-NMR spectroscopy. Strong alkaline deacylation of the LPS resulted in two core-lipid A backbone undecasaccharide pentakisphosphates in the ratio approximately 2.5 : 1, which corresponded to outer core glycoforms 1 and 2 terminated with either L-rhamnose or 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo), respectively. Mild acid degradation of the LPS gave the major glycoform 1 core octasaccharide and a minor truncated glycoform 2 core heptasaccharide, which resulted from the cleavage of the terminal Kdo residues. The inner core of P. syringae is distinguished by a high degree of phosphorylation of L-glycero-D-manno-heptose residues with phosphate, diphosphate and ethanolamine diphosphate groups. The glycoform 1 core is structurally similar but not identical to one of the core glycoforms of the human pathogenic bacterium Pseudomonas aeruginosa. The outer core composition and structure may be useful as a chemotaxonomic marker for the P. syringae group of bacteria, whereas a more conserved inner core structure appears to be representative for the whole genus Pseudomonas.

N-acetylmuramic acid as capping element of alpha-D-fucose-containing S-layer glycoprotein glycans from Geobacillus tepidamans GS5-97T

Geobacillus tepidamans GS5-97(T) is a novel Gram-positive, moderately thermophilic bacterial species that is covered by a glycosylated surface layer (S-layer) protein. The isolated and purified S-layer glycoprotein SgtA was ultrastructurally and chemically investigated and showed several novel properties. By SDS-PAGE, SgtA was separated into four distinct bands in an apparent molecular mass range of 106-166 kDa. The three high molecular mass bands gave a positive periodic acid-Schiff staining reaction, whereas the 106-kDa band was nonglycosylated. Glycosylation of SgtA was investigated by means of chemical analyses, 600-MHz nuclear magnetic resonance spectroscopy, and electrospray ionization quadrupole time-of-fight mass spectrometry. Glycopeptides obtained after Pronase digestion revealed the glycan structure [-->2)-alpha-L-Rhap-(1-->3)-alpha-D-Fucp-(1-->](n=approximately 20), with D-fucopyranose having never been identified before as a constituent of S-layer glycans. The rhamnose residue at the nonreducing end of the terminal repeating unit of the glycan chain was di-substituted. For the first time, (R)-N-acetylmuramic acid, the key component of prokaryotic peptidoglycan, was found in an alpha-linkage to carbon 3 of the terminal rhamnose residue, serving as capping motif of an S-layer glycan. In addition, that rhamnose was substituted at position 2 with a beta-N-acetylglucosamine residue. The S-layer glycan chains were bound via the trisaccharide core -->2)-alpha-L-Rhap-(1-->3)-alpha-L-Rhap-(1-->3)-alpha-L-Rhap-(1--> to carbon 3 of beta-D-galactose, which was attached in O-glycosidic linkage to serine and threonine residues of SgtA of G. tepidamans GS5-97(T).

Full structure of the lipopolysaccharide of Pseudomonas aeruginosa immunotype 5

The lipopolysaccharide (LPS) of the opportunistic human pathogen Pseudomonas aeruginosa immunotype 5 was delipidated by mild acid hydrolysis, and the products were separated by high-performance anion-exchange chromatography and analyzed by ESI MS and NMR spectroscopy. LPS species of three types were found, including those with an unsubstituted core and the core substituted with one O-polysaccharide repeating unit or with an O-polysaccharide of a variable number of repeating units. The core region is highly phosphorylated, the major species containing two monophosphate groups and one ethanolamine diphosphate group. Based on these and published data on the O-polysaccharide structure, the full structure of the LPS of P. aeruginosa immunotype 5 was established.

Structure of theBordetella trematumLPS O-chain subunit

Analysis of the O-chain subunit of the lipopolysaccharide (LPS, endotoxin) isolated from Bordetella trematum, a recently identified human pathogen, was undertaken. The polysaccharide (PS) moiety was shown to contain only two O-chain subunits, which differed in the anomeric bond of their first sugar. A trisaccharide fragment resulting from the cleavage of a FucNAc glycosidic bond was isolated after treatment of the PS with anhydrous HF. Nitrous deamination of the LPS led to the release of the following heptasaccharide corresponding to two trisaccharide subunits linked to an anhydromannitol residue. beta-ManNAc3NAmA-(1-4)-beta-ManNAc3NAmA-(1-3)-alpha-FucNAc-(1-4)-beta-ManNAc3NAmA-(1-4)-beta-ManNAc3NAmA-(1-3)-beta-FucNAc-(1-6)-2,5-anhManol.

Molecular basis of bacterial outer membrane permeability revisited

Gram-negative bacteria characteristically are surrounded by an additional membrane layer, the outer membrane. Although outer membrane components often play important roles in the interaction of symbiotic or pathogenic bacteria with their host organisms, the major role of this membrane must usually be to serve as a permeability barrier to prevent the entry of noxious compounds and at the same time to allow the influx of nutrient molecules. This review summarizes the development in the field since our previous review (H. Nikaido and M. Vaara, Microbiol. Rev. 49:1-32, 1985) was published. With the discovery of protein channels, structural knowledge enables us to understand in molecular detail how porins, specific channels, TonB-linked receptors, and other proteins function. We are now beginning to see how the export of large proteins occurs across the outer membrane. With our knowledge of the lipopolysaccharide-phospholipid asymmetric bilayer of the outer membrane, we are finally beginning to understand how this bilayer can retard the entry of lipophilic compounds, owing to our increasing knowledge about the chemistry of lipopolysaccharide from diverse organisms and the way in which lipopolysaccharide structure is modified by environmental conditions.

Structure of a highly phosphorylated lipopolysaccharide core in the ΔalgC mutants derived from Pseudomonas aeruginosa wild-type strains PAO1 (serogroup O5) and PAC1R (serogroup O3)

Lipopolysaccharides (LPS) were isolated from rough-type mutant strains of Pseudomonas aeruginosa (Delta algC) derived from wild-type strains PAO1 (serogroup O5) and PAC1R (serogroup O3). Structural studies of the LPS core region with a special focus on the phosphorylation pattern were performed by 2D NMR spectroscopy, including a 1H,(31)P HMQC-TOCSY experiment, MALDI-TOF MS, and Fourier-transform ion cyclotron resonance ESIMS using the capillary skimmer dissociation technique. Both LPS were found to contain two residues each of 3-deoxy-D-manno-oct-2-ulosonic acid (Kdo) and L-glycero-D-manno-heptose (Hep), one residue of N-(L-alanyl)-D-galactosamine and one O-carbamoyl group (Cm) on the distal Hep residue. The following structures of a tetrasaccharide trisphosphate from strain PAC1R Delta algC and that carrying an additional ethanolamine phosphate group (PEtN) from strain PAO1 Delta algC were elucidated: [carbohydrate structre: see text] where R=P in PAC1R Delta algC and PPEtN in PAO1 Delta algC. To our knowledge, in this work the presence of ethanolamine diphosphate is unambiguously confirmed and its position established for the first time in the LPS core of a rough-type strain of P. aeruginosa. In addition, the structure of the complete LPS core of wild-type strain P. aeruginosa PAO1 was reinvestigated and the position of the phosphorylation sites was revised.

Structure of the lipopolysaccharide of Pseudomonas aeruginosa O-12 with a randomly O-acetylated core region

The lipopolysaccharide of Pseudomonas aeruginosa O-12 was studied by strong alkaline and mild acid degradations and dephosphorylation followed by fractionation of the products by GPC and high-performance anion-exchange chromatography and analyses by ESI FT-MS and NMR spectroscopy. The structures of the lipopolysaccharide core and the O-polysaccharide repeating unit were elucidated and the site and the configuration of the linkage between the O-polysaccharide and the core established. The core was found to be randomly O-acetylated, most O-acetyl groups being located on the terminal rhamnose residue of the outer core region.

Structure of the biological repeating unit of the O-antigen of Pseudomonas aeruginosa immunotype 4 containing both 2-acetamido-2,6-dideoxy-D-glucose and 2-acetamido-2,6-dideoxy-D-galactose

A phosphorylated core-lipid A backbone oligosaccharide that carries a disaccharide remainder of the first O-antigen repeating unit was derived by strong alkaline degradation following mild hydrazinolysis of the lipopolysaccharide of Pseudomonas aeruginosa immunotype 4 (serogroup O-1). The structure of the oligosaccharide was determined using ESI MS and NMR spectroscopy and it was demonstrated that 2-acetamido-2,6-dideoxy-D-glucose is the first monosaccharide of the O-polysaccharide that is linked to the LPS core. These data define the structure of the biological repeating unit of the O-antigen.

Protection against fatal Pseudomonas aeruginosa pneumonia in mice after nasal immunization with a live, attenuated aroA deletion mutant

Studies of immunity to Pseudomonas aeruginosa have indicated that a variety of potential immunogens can elicit protection in animal models, utilizing both antibody- and cell-mediated immune effectors for protection. To attempt to optimize delivery of multiple protective antigens and elicit a broad range of immune effectors, we produced an aroA deletion mutant of the P. aeruginosa serogroup O2/O5 strain PAO1, designated PAO1deltaaroA. Previously, we reported that this strain elicits high levels of opsonic antibody directed against many serogroup O2/O5 strains after nasal immunization of mice and rabbits. Here, we assessed the protective efficacy of immunization with PAO1deltaaroA against acute fatal pneumonia in mice. After active immunization, high levels of protection were achieved against an ExoU-expressing cytotoxic variant of the parental strain PAO1 at doses up to 1,000-fold greater than the 50% lethal dose. Significant protection against PAO1 and two of four other serogroup O2/O5 strains was also found, but there was no protection against serogroup-heterologous strains. The serogroup O2/O5 strains not protected against were killed in opsonophagocytic assays as efficiently as the strains with which protection was seen, indicating a lack of correlation of protection and opsonic killing within the serogroup. In passive immunization experiments using challenge with wild-type PAO1 or other noncytotoxic members of the O2/O5 serogroup, there was no protection despite the presence of high levels of opsonic antibody in the mouse sera. However, passive immunization did prevent mortality from pneumonia due to the cytotoxic PAO1 variant at low-challenge doses. These data suggest that a combination of humoral and cellular immunity is required for protection against P. aeruginosa lung infections, that such immunity can be elicited by using aroA deletion mutants, and that a multivalent P. aeruginosa vaccine composed of aroA deletion mutants of multiple serogroups holds significant promise.